Large computer networks, such as Wide Area Networks (WAN) or Internet backbone systems often incorporate various types of network devices (nodes) and network protocols. The SONET (Synchronous Optical Network) system is a high bit-rate fiber-optic based transport system that has become a well established standard for linking low and high-speed devices such as switches and multiplexers in wide-scale networks.
SONET uses a self-healing ring architecture of two or more transmission paths between network nodes. If there is a break or error condition in one line, the other line is utilized to provide automatic protection against failures. Various ring topologies are possible for SONET systems, including two-fiber and four-fiber bidirectional line switched protected rings (BLSR) and two-fiber unidirectional path switched ring (UPSR). SONET is bandwidth flexible and operates at line rates from VT1.5 to OC-768 (and potentially higher) regardless of network size. In general, for SONET systems, line rate specific OC-n network elements are utilized at each point node location. The data traffic is fitted into specific bandwidth slots, and signal concatenation schemes are used to extend payload envelopes for supporting dense data transfers. A SONET layer is comprised of section, line and path components. Regardless of the data transfer rate, a SONET frame contains only one section and line layer with their corresponding overhead. At the path layer, however, the potential number of STS frames and their corresponding overhead depends on the data transfer rate of the Optical Carrier (OC). An OC-n, where n is typically equal to 1, 3, 12, 48, 192 and 768, consists of potentially n STS frames. The exact number of STS frames depends on the type and number of concatenated STS frames used (e.g., STS-3c or STS-12c). If none are concatenated, the number of STS frames is equal to n, otherwise the number of frames is smaller than n.
SONET networks provide various protection mechanisms. Examples include APS (Automatic Protection Switching) 1:1 protection that provides line redundancy, and APS 1+1 protection that provides card and line redundancy. These schemes involve monitoring individual transmission lines in the SONET network and selecting the one with the better quality. These line layer protection schemes generally do not increase in complexity with the data transfer rate, since only a single line is monitored in all cases. For the path layer protection, a common protection mechanism is UPSR (unidirectional path switched ring). As the data rate increases, there are more STS paths to monitor. A common UPSR protection scheme relies on a selector circuit placed between network node for each counter-rotating ring of the SONET network. The selector monitors STS signals from one ring and STS signals from the other (counter-rotating) ring and compares the two signals. Each STS signal includes various quality measurement data items, such as bit error rate, and so on. The selector examines this information from each STS signal and selects the signal of higher quality. For the above simple example, the user sees only one STS signal, even though the ring transmits or passes two STS signals. However, as the data rate increases, the number of STS selection operations also increases. Thus, for OC-3, the selector circuit selects among three pairs of STS signals (three pair are received from one ring and the other three pair are received from the counter rotating ring), for OC-12, the selector circuit selects among twelve pairs of STS signals, and for OC-192, the selector circuit selects among 192 pairs of STS signals. This selection operation can impose a great processing load on the central processor, especially at higher data rates. In practical applications, the selection operation must be performed within a certain maximum time period, for example selections must be made within 50 milliseconds. For high data rate networks, the UPSR selection requirements may exceed the bandwidth availability of the central processing unit.
A further disadvantage of present SONET networks is that typical cross-connect or multiplexer devices are not capable of automatic reconfiguration in the event of failure or user command conditions. Cross-connect devices are intended to provide switching functionality from working rings to standby rings in the event of failure or forced switching conditions. Most present cross-connect devices must be programmed or manually set to switch between working and protection rings. For example, in most present software switching solutions, when a defect is detected, a signal is sent to a central processing unit (CPU). The CPU then re-provisions the cross-connect to switch from the working ring to the protection ring (or from the protection ring to the working ring). An example of this type of prior art switching method utilizing a CPU to re-provision a network cross connect is illustrated in FIG. 1. In FIG. 1, a CPU 1106 is coupled between monitor circuits 1102 and 1104 and cross-connect 1108. The STS channel outputs are transmitted from the monitor circuits to the CPU. Depending upon the relative quality of the input STS signals, the CPU programs the cross-connect 1108 to select the higher quality STS channel. This present CPU-based switching method adds complexity and increases switching latencies in many SONET applications.
What is needed, therefore, is a UPSR protection mechanism for SONET networks that provides automatic reconfiguration of network cross connect devices in the event of the detection of error conditions for a large number of STS frames, and that eliminates the CPU from the ring selection process.